This invention relates to glass compositions and especially to glass for filters having steep absorption edges in the wavelength range .lambda. = 455 to 850 nm combined with an internal transmittance of .gtoreq. 0.80, and which have good chemical stability, increased Knoop hardness, and greatly reduced viscosity in the melting range, thus facilitating the production of glass having a high optical quality.
Yellow, orange, red, and infrared glass filters having a selectively decreasing absorption band in the spectral range from .lambda. 455 to 850 nm combined with an internal transmittance of .gtoreq. 0.80 are known, for example from DT-OS 20 26 485 and from U.S. Pat. No. 3,773,530, said glasses being obtained by "colloidal coloring" CdS + S + Se + Te into SiO.sub.2 -ZnO-CaO-BaO alkali metal oxide glasses followed by an annealing step. These colored glasses, however, have one or more of the following disadvantages:
1. Because of the excessively high viscosity of certain base glasses which have good chemical resistance, it is not possible to ensure that the incorporation of the coloring system can be conducted within the homogenization range of the base glass. Consequently, bubble quality may be impaired owing to post-gasification, and this is extremely detrimental to the cord quality of steep edge filters, because effective homogenization or an intensive stirring technique cannot be applied in these cases. Furthermore, an excessively high viscosity of a glass melt results in the displacement of the absorption edge in the infrared direction, this displacement being regarded as completed at a wavelength of 640 nm.
2. Glasses of a limited optical quality and having high contents of alkali metal oxide and ZnO tend to develop flaws during processing, cementing, and polishing. Whereas the stable incorporation of the coloring substances is possible with glass systems having a high alkali content if the viscosity values are above 3500 poises, in this viscosity range, the intensive mechanical stirring techniques intended to achieve good optical homogeneity of glass pastes are likely not to function as desired.
3. With regard to hardness properties, all glasses previously known, specifically having "colloidal coloring", come within a limit of .+-. 20 kp/mm.sup.2 (with a load of 200p), starting from the mean Knoop hardness value of 420 kp/mm.sup.2 ; otherwise they show considerable lightening of the absorption background. This factor substantially restricts the field of application of these steep edge filters in photometry.
4. The production of known colored glasses with "colloidal coloring" is subject to very strict melting conditions. Fluctuations with respect to the steepness and position of the absorption edge must be expected from one melt to another. Also, because of their excessive viscosity in the melting range, they are not suitable for the continuous production of steep edge filter glasses of optical quality.